Three-way catalysts for simultaneously eliminating hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) from exhaust gases emitted from internal-combustion engines of automobiles and the like generally comprise a platinum group element such as platinum, rhodium or palladium, in combination with cerium oxide having oxidation-reduction performance (oxygen storing effect) and serving to improve the low-temperature catalyst activity.
However, at high temperatures exceeding 800.degree. C. the oxidation-reduction performance of catalysts containing a platinum group element and cerium oxide markedly reduce as a result of grain growth of cerium oxide. In this connection, a number of proposals have been made to add an oxide of a rare earth element and/or of zirconium to thereby inhibit the grain growth (crystallization) of cerium oxide and maintain the oxidation-reduction performance thereof up to high temperatures (for example, Japanese Kokai Tokkyo Koho S64-58347; Japanese Kokai Tokkyo Koho S63-116741). It has also been disclosed that a mixed oxide containing cerium oxide and zirconium oxide be prepared and use the same as a catalyst (for example, Japanese Kokai Tokkyo Koho S62-168544; Japanese Kokai Tokkyo Koho H01-281144; Japanese Kokai Tokkyo Koho H04-284847). It is thus known that mixed oxides containing cerium oxide and zirconium oxide can be utilized as promoters or catalyst supports for exhaust gas treatment catalysts for improving the heat resistance or oxidation-reduction performance of platinum group elements, which are active catalyst components.
As regards the mixed oxides containing cerium oxide and zirconium oxide, a number of methods have been disclosed of improving the characteristics, for example thermal stability of specific surface area, oxidation-reduction performance etc. of the mixed oxides in order to improve the functions thereof as promoters or catalyst supports for exhaust gas treatment catalysts (for example, Japanese Kokai Tokkyo Koho H04-55315; Japanese Patent No. 2,698,302; Japanese Kokai Tokkyo Koho H05-286722; Japanese Kokai Tokkyo Koho H09-278444).
Meanwhile, today, environmental safeguard measures are being pushed forward and, with the recent tightening of legal regulations with regard to exhaust gases, the endurance temperature required with regard to the thermal stability of specific surface area and the deterioration of oxidation-reduction performance of a promoter (co-catalyst) and/or catalyst support is now as high as about 1,000.degree. C.
However, in the case of the conventional cerium oxide-zirconium oxide mixed oxides, the thermal stability of specific surface area is at most about 20 m.sup.2 /g after heat treatment at 1,000.degree. C. and, even when the mixed oxides are not heat-treated, the oxidation-reduction performance is such that the hydrogen consumption is at most about 5 cc/g.
Furthermore, it is known that, under certain conditions of running of automobiles (for example during 10-mode test (emission test mode) in Japan), most of the total emission of unpurifiable hydrocarbons is emitted at lower temperatures than the temperature at which the catalysts can start functioning. The temperature at which a catalyst begins to function is referred to as T.sub.50 (temperature at which the purification degree reaches 50% of the maximum degree of purification) and it is known that the increase in the content of a noble metal such as platinum, which is an active component of the catalyst, results in shifting of the T.sub.50 to the lower temperature side. However, the increase in the content of such a noble metal not only results in an increase in production cost but also in exhaustion of resources. Therefore, it is also necessary to improve the catalyst performance in a low temperature region without relying on the increase in noble metal content.
On the other hand, a general method of unifying cerium oxide and zirconium oxide homogeneously comprises adding a base to a cerium ion- and zirconium ion-containing aqueous solution and recovering the double salt precipitate (for example, Japanese Kokai Tokkyo Koho H09-278444).
However, the double salt precipitate obtained by the above method occurs as a gel-like bulky hydroxide with a high content of water. Therefore, a filtration or solid-liquid separation step is essential for the elimination of impurities. The rate of processing per lot is inevitably low and a vast quantity of energy is required for conversion to oxides. The conventional methods of production thus cannot be said to be suitable for industrial scale production.
Accordingly, it is a primary object of the present invention to provide a mixed oxide particularly excellent in thermal stability of specific surface area and in oxidation-reduction performance on an industrial scale. Another object of the invention is to provide a catalyst material for exhaust gas elimination which contains said mixed oxide.